Downhole pressure measurements are essential when drilling for hydrocarbon recovery. During the drilling process, geological pressure data are collected to tailor drilling parameters and the construction of the well. After the well is drilled and production starts, pressure is continuously monitored for reservoir management. Accurate measurement of pressure is therefore the key to optimize recovery and reduce risk throughout the entire life of a hydrocarbon well. Thus, we need an accurate and cost-effective pressure sensor for downhole measurements.
Pressure sensors usable in hydrocarbon wells must be able to withstand harsh conditions and remain accurate, stable and reliable for weeks during a measurement period. In particular, such sensors must be able to withstand temperature ranging from −50° C. to 250° C. and pressure up to 200 MPa (around 2000 atmospheres) while maintaining an accuracy of better than 0.1%, and desirably 0.01%, of the full-scale pressure.
Two types of pressure sensors are commonly used for downhole applications. The first type is the resonant quartz pressure sensor. In U.S. Pat. No. 3,617,780, one example of resonant quartz pressure sensor is described wherein a crystalline quartz cylinder closed at both ends is immersed in a fluid which communicates with the external pressure to be measured via an isolation diaphragm or a bellow. A crystalline quartz plate spans across the vacuum-sealed cavity inside the cylinder. The plate resonance is excited and detected via the piezoelectric effect. The plate resonant frequency, which varies with the hydrostatic pressure on the cylinder wall, is a measure of the external pressure. Constructed almost entirely out of crystalline quartz and being a mature technology, resonant quartz pressure sensors have achieved the highest benchmark for accuracy, stability and reliability for downhole pressure measurements to date. However, they tend to be very expensive.
The second type of downhole pressure sensors is based on sapphire. In U.S. Pat. No. 5,024,098, a sapphire pressure sensor is described wherein a sapphire cell is immersed in a fluid which communicates with the external pressure to be measured via an isolation diaphragm. The cell deforms under fluid pressure and the resulting strains are measured by strain gauge elements disposed on a planar surface of the sapphire cell. While reliable and rugged for downhole applications, sapphire pressure sensors are in general not as stable and accurate as resonant quartz pressure sensors and they are also quite expensive. In case silicon strain gauge elements are employed, accuracy and stability could be affected by the excessive temperature coefficients of resistance and piezoresistive effect in silicon. On the other hand, if non-silicon strain gauge elements, for example, metallic alloys, are used their low gauge factor and therefore sensitivity can result in the undesirable amplification of temperature and other measurement errors. In any case, mismatch in the thermal expansion coefficients between sapphire and the strain gauge material creates further temperature errors.
The majority of sensors in use today are of the micro-electro-mechanical system (MEMS) type. MEMS based sensors are typically realized with silicon micromachining that originated from integrated circuit fabrication and still shares many of its processing technologies. In addition, there are a few unique processes specifically tailored toward the fabrication of 3-dimensional microstructures. These include double-side photolithography, deep reactive ion etching (DRIE), and wafer bonding to name a few. Silicon has superb mechanical properties not unlike quartz and sapphire, for example, high hardness, high modulus of elasticity, high ultimate strength, and is perfectly elastic up to the fracture point. Moreover, precision microstructures are much easier to fabricate in silicon than in quartz or sapphire. With demonstrated advantages that include low cost, small size, high accuracy, high reliability, and high stability, diaphragm-type silicon MEMS pressure sensors have become the dominant type of pressure sensors in use for automotive, medical, industrial and consumer electronics applications.
Despite their huge success, MEMS pressure sensors have not been widely adopted for downhole applications. There are a few problems that must be overcome. In particular, an improved mechanical design over the conventional diaphragm-type pressure sensors is preferable to handle the very high pressure. Furthermore, there needs to be a better means to deal with the various temperature coefficients and instabilities for overall improved measurement accuracy at high temperature. Accordingly, a need presently exists for an improved MEMS pressure sensor having a high degree of accuracy and reliability for the cost effective measurement of high pressure under high temperature conditions typical of downhole applications.